1. Field of the Invention
The present invention relates to optical recording medium in which information is recordable, reproduceable and rewritable at a high density and a high speed, using a laser beam irradiator.
2. Description of the Related Art
In an optical recording medium, a laser beam is irradiated locally to a recording material and then a difference in optical property generated thereby is used as a recording state. Use of a material having reversible change in this optical property enables rewriting the information which has been recorded. Generally, as a rewritable optical recording medium, a magneto-optical recording medium and a phase change optical recording medium are well known. These optical recording media enable recording mass of information as well as rewriting and reproducing the information at high speed simultaneously. These optical media are also excellent in portability. Accordingly, a demand has been increasing to produce these optical media for more capacity at a higher speed.
A phase change optical recording medium takes advantage of a difference of reflected light to a light having a specific wavelength between the crystalline and the amorphous as a recording state. Modulating output power of the laser enables erasing and rewriting the recorded information simultaneously. The modulation accordingly allows the phase change optical recording medium to be rewritten the information signal at a high speed and with ease.
FIG. 4 shows an example of a conventional structure of layers of a phase change optical recording medium. As shown in FIG. 4, a conventional phase change type optical recording medium is constituted by a substrate 1, and a protective layer 2, a recording layer 4, a protective layer 8 and a reflective layer 6, all of which are sequentially formed on the substrate 1. The substrate 1 is made of resins such as polycarbonates (PC) and polymethylmethacrylates (PMMA), or glasses. The substrate 1 guide grooves for guiding a laser beam are formed thereon. The recording layer 4 has some states having different optical properties, and comprises a substance that can reversibly change the states. In case of a rewritable phase change type optical recording medium, materials for the recording layer 4 include chalcogenide whose main components are Te or Sb such as materials having main components of Te—Sb—Ge, Te—Sn—Ge, Te—Sb—Ge—Se, Te—Sn—Ge—Au, Ag—In—Sb—Te, In—Sb—Se, In—Te—Se, or the like.
The reflective layer 6 comprises metals such as Au, Al, and Cr, or alloys thereof. The reflective layer 6 is prepared for purposes of dissipating heat effectively and of effective light absorption in the recording layer 4. Although not shown in the FIG. 4, an over coating layer is provided on the reflective layer 6 in order to prevent oxidization, corrosion, adhesion of dust or the like. Alternatively, a dummy substrate may be provided on the reflective layer 6, using the ultraviolet radiation curing resins as an adhesive.
The protective layers 2 and 8 play a role of preventing oxidation, evaporation, and deformation of the materials for the recording layer 4. Controlling the thickness of the protective layers 2 and 8 enables adjusting light absorption of a recording medium and a difference of reflection ration between a recording portion and an erasing portion. Accordingly, the protective layers 2 and 8 play roles of controlling optical properties of the recording medium. The materials for the protective layers 2 and 8 are required to exhibit excellent adhesion properties to the recording layer 4 and the substrate 1, in addition to meeting the requirements above. The protective layers 2 and 8 are required to be a film having excellent weathering resistance that does not cause cracklings. When contacted with the recording layer 4, the protective layers 2 and 8 are required to be composed of materials that do not affect optical change in the recording layer 4.
Examples of the materials for the protective layers 2 and 8 include sulfides such as ZnS, or the like; oxides such as SiO2, Ta2O5, Al2O3, or the like; nitrides such as Ge—N, Si3N4, Al3N4 or the like; nitrogen oxides such as Ge—O—N, Si—O—N, Al—O—N, or the like. The examples further include dielectrics such as carbides and fluorides, or the like. These may be used in suitable combination of two or more. Of these, ZnS—SiO2 is widely used.
Conventionally, overwriting distortion occurs. The overwriting distortion is caused by a state in which a rewritten mark slightly shears. The overwriting distortion occurs because the temperature rises differently depending on a state of recording layer 4 between in an amorphous state and in a crystalline state. A portion before rewriting requires latent heat to phase-change the portion from a crystalline state to an amorphous state, when the portion before rewriting is in a crystalline state. On the other hand, when the portion before rewriting is in an amorphous state, the latent heat is not required. Therefore, excess heat amorphousizes the recording layer 4 more than predetermined.
When “Aa” expresses a light absorption of the recording layer 4 in an amorphous state, and “Ac” expresses a light absorption of the recording layer 4 in a crystalline state, “Ac/Aa” may be maintained in 1 or more in order to avoid the overwriting distortion, which enables adjusting light absorption. Accordingly rise in temperature at an amorphous portion of the recording layer can be assisted. The temperature at the marked portion after rewriting rises uniformly. Mark distortion is hence less likely to occur.
Some methods have been proposed to realize a relation of: Ac/Aa>1. For example, “Ra,” which is a reflection rate of an amorphous state, is determined to be higher than “Rc,” which is a reflection rate of a crystalline state, so as to satisfy the relation of: “Rc<Ra.” In this case, even if a difference, “Ra−Rc,” of reflection ratios between an amorphous state and a crystalline state is large, a value of Ac/Aa may still be large. Specifically, in FIG. 4, another layer is formed between the substrate 1 and the protective layer 2, and the layer has a certain optical constant, hence the relation of “Rc<Ra” can be satisfied.
Even if “Rc” and “Ra” meet the relation of “Rc>Ra,” the relation, “Ac/Aa>1,” may still be attained. In this case, the optical recording medium employs either light-transmittance structure, or light-absorbing structure. The light-transmittance structure creates transmittance in the optical recording medium. When “Tc” expresses transmittance of amorphous recording layer, and “Ta” expresses transmittance of crystalline recording layer, “Tc” and “Ta” each satisfy the relation of 0<Tc<Ta. On the other hand, in the light-absorbing structure, a layer that absorbs a light is provided in the optical recording medium. The light absorption in the layer that absorbs a light satisfies a relation of 0<Ac2<Aa2, when the Aa2 expresses an absorption at the layer that absorbs a light in an amorphous state, and Ac2 expresses an absorption in a crystalline state. Specifically, in a case of the light-transmittance structure, the reflective layer 6 may be thinned so as to attain light-transmittance, as shown in FIG. 4. In a case of the light-absorbing structure, for instance in FIG. 4, a layer that absorbs a light may be provided between the reflective layer 6 and the protective layer 8.
An optical recording medium having such a relation of reflection rate as Rc<Ra is advantageous since the optical recording medium is more likely to have a structure that satisfies a relation of: Ac/Aa>1. The optical recording medium, on the other hand, is disadvantageous in causing noise at reproducing a signal, as the sum of reflection rate at the amorphous portion and the crystalline portion are considerably larger than that of an optical recording medium having such a relation of reflection rate as Rc>Ra. The optical medium having such a relation of reflection rate as Rc>Ra is less likely to have a disadvantages like noise, but it is still disadvantageous in having a large value for Ac/Aa. Accordingly, it is preferable to choose the structures depending on the necessity.
Some improvement has been proposed conventionally for the structure of a light-transmittance optical recording medium that satisfies the relations of both “Rc>Ra” and “0<Tc<Ta.” For example, Japanese Patent Application Laid-Open (JP-A) No. 08-50739 discloses a technique in which a recording layer and a reflective layer having a light-transmittance properties are provided. In this technique, the reflective layer is provided in contact with a thermal dissipating layer that helps thermal diffusion of the reflective layer in an optical recording medium that employs light-transmission. The JP-A No. 08-50739 does not state any technique to give optical effects to the thermal dissipating layer, and describes that the thickness of the thermal dissipating layer may be suitably selected as long as it does not prevent the optical structure or design.
JP-A No. 09-91755 discloses a technique in which a dielectric layer is provided on a reflective layer in an optical recording medium having light-transmittance. However, in this case, the dielectric layer is formed in order to reduce phase difference. The JP-A No. 09-91755 does not states the thermal effects derived from the dielectric layer, neither states the optical effects derived from controlling the thickness of the dielectric layer.
The JP-A No. 03-157830 discloses an optical recording medium having two recording layer structures, which has been known as the modified optical recording medium having a light-transmittance structure. In order to attain a larger capacity of the optical recording medium, a transparent separation layer is provided between the two recording layer structures. A laser is irradiated from only one direction, and the laser transmits both of the two recording layer structures. With this technique, a density of recording may become more intense, hence a capacity of the optical recording medium becomes larger as a whole.
An optical recording medium having a light-transmittance structure is advantageous from a viewpoint of having less excess heat therein. An optical recording medium having a light-transmittance structure is hence desirable from a viewpoint of repeating properties and adjacent erasing properties (properties to erase an adjacent tracks; tracks that has been recorded are diffused to an adjacent track, and the signals recorded adjacent to the tracks are erased). Having a thin reflective layer, the recording layer may not be rapidly cooled down after heated. Therefore, a mark may be formed with difficulty. Especially, in a structure satisfying a relationship of Rc>Ra, it was fundamentally difficult to set a value of Ac/Aa very large. The optical recording medium having two recording layer structures has conventionally required the recording layer to be thin in order to attain a sufficient light-transmittance, when the optical recording medium having two recording layer structures is placed in a direction of laser-irradiation.
However, crystallization becomes difficult in the thin recording layer. High light transmittance was unable to compatible with high erasing rate or high erasing properties. There are very few techniques to improve repetitive recording properties of a light-transmittance optical recording medium. A demand has been made on improving the repetitive recording properties.